Because of their wide range of physical and chemical properties and unique structural characteristics, chalcogenides (sulfides, selenides and tellurides) of metals are widely used in industrial applications such as catalysis, lubrications, battery fabrication, ionic conduction, refractories, pigments, sensors, and optical and magnetic devices. Among the various preparation methods of metal chalcogenides, the simplest method to date is based on direct reactions between chalcogen elements and elemental metals at elevated temperatures. Most of the other methods utilize metathetical reactions of metal source compounds and binary chalcogenides such as H2S, H2Se, H2Te, CS2, Y2S3, Na2S, Na2S2, K2S and K2S2. The reactions can take place in solid, liquid or gaseous state as well as in solution.
Discovery and utilization of new chalcogen sources are of great value for the development of new methodologies that may overcome the limitation of the current methods of chalcogenide preparation. For example, the synthesis of metal polysulfides has not been straightforward by the existing methods because many polysulfides decompose at elevated temperatures and the sources of the polyanions are relatively scarce. H2S alone is not an efficient source of S0, and the thermodynamics of the thermal decomposition of H2S are not favorable at low temperatures. For instance, at temperatures below 550° C. the equilibrium concentration of sulfur is less than 1%, and even at 900° C. it is only 13%. Reactions with elemental sulfur often require a pressurized reaction container and/or a multi-step procedure. More recently, solid-state metathesis and/or solution methods have been used for the preparations of disulfides of Fe, Co, Ni, La and Pr by employing Na2S2, K2S2 or Na2S5. FeS2 and CoS2 have also been prepared at higher temperatures by reacting H2S with starting materials that contain the corresponding metal ions of high oxidation states, but the problem of incomplete reactions and/or of impurities still remains.
Utilization of boron chalcogenides has not been previously reported for preparation of inorganic metal chalcogenides. That is, even the most heavily studied boron chalcogenides to date, boron sulfides (B2S3, BS2, and nonstoichiometric compounds with intermediate B/S ratios), have not been used in solid state syntheses other than for preparations of thioborates, and their use for sulfidation has been reported only in organic or organometallic reactions, and even there only sporadically. The sulfides do not have a well-defined melting point, but they begin to sublime at about 300° C. under vacuum from our experience as well as that of others. Previously stoichiometric B2S3 (s) vaporizes congruently to give B2S3(g) and its polymers, while sulfur-rich samples vaporize incongruently into (BS2)n(g) and (B2S3)n(g). The corrosive nature of the gaseous boron sulfides reported in the literature is probably the main reason for their scarce use in solid state reactions; and indeed the silica reaction vessels need to be heavily carbon-coated for the synthesis of alkali thioborates, which is carried out typically above 600° C. for several days.
However, for sulfidation reactions, the corrosiveness of the gaseous boron sulfides could be advantageous, particularly under low-temperature reaction conditions which are often required in preparation of nanostructural materials. Furthermore, the boron sulfides on the sulfur-rich end may allow access to the polysulfide compounds that exist only at low temperatures and decompose at elevated temperatures.